Waterborne microbial infection is a significant public health concern, particularly in developing countries where sanitation facilities are inadequate. According to the World Health Organization, over 130,000 cases of cholera (Vibrio cholerae) and 17 million cases of Typhoid fever (Salmonella typhi) occur annually. It is remarkable that these and other disease-causing microbes can thrive both in natural aquatic reservoirs and inside the human body. Bacterial survival across such varied environments requires robust sensory systems that can rapidly modulate cell physiology. This application focuses on the molecular, genetic, and biochemical characterization of a novel environmental sensory system in the model freshwater bacterium, Caulobacter crescentus. Caulobacter are not typically pathogenic, but have the ability to adapt to diverse aquatic environments ranging from distilled water to wastewater. In addition to existing as free-living cells, Caulobacter can also enter a multicellular developmental state known as a biofilm. The cellular signaling networks that control physiological and developmental transitions in bacteria largely use two-component signal transduction systems, consisting of histidine protein kinases and their cognate response regulator proteins. We have evidence that C. crescentus contains a two-component signaling system, encoded by the lovK and lovR genes, that detects light and regulates cellular attachment, a necessary step in biofilm development. Specifically, lovK encodes a photoresponsive histidine kinase and the lovR gene encodes a single-domain response regulator. Related photosensor kinases are found in human and plant pathogens such as Brucella melitensis, Pseudomonas syringae, and Xanthomonas campestris. Light is a ubiquitous environmental signal that can act as a proxy for whether a cell is inside or outside a host organism. Thus, light may be an important signal regulating biofilm formation and pathogenicity in select bacterial species. Caulobacter is an excellent model system in which to assess the functional role of light and two-component signaling in biofilm development because there are abundant molecular, genetic, and genomic tools available. Public Health Relevance I will determine the effects of light quality and quantity on the regulation of cellular attachment and define downstream genes in the LovK/LovR signaling pathway in Caulobacter. Through this work we will gain a better understanding of the role the LovK/LovR two-component system and the ubiquitous environmental signal, sunlight, in the regulation of bacterial physiology and development.